The invention relates generally to measurement of components in a gas stream, and more particularly to an electrochemical apparatus and method for measuring the concentration of gaseous oxides in a gas mixture.
Various devices and methods have been described for determining the concentration of oxides of nitrogen (NOx, for example, N2O, NO and NO2), oxides of carbon (COx for example, CO and CO2), oxides of sulfur (SOx for example, SO2 and SO3), and other oxide compounds in a gas mixture. Such gases may include gaseous oxygen (O2), nitrogen (N2), other inert gases, as well as combustible gases such as H2 and various hydrocarbons.
Most modern automobiles use an O2 sensor which is disposed in the exhaust system together with an on board computer to control the amount of fuel injected for combustion. Usually, the computer only utilizes oxygen sensor data (xe2x80x9cclosed loopxe2x80x9d mode) under cruise conditions to improve efficiency. The O2 sensor outputs a voltage when the oxygen content of the exhaust gasses falls below the norm for the atmosphere. The voltage range is generally from 0 to 1 volt. The O2 sensor is not sensitive to gases other than O2.
Oxygen in the air is consumed when fuel burns. Accordingly, increasing the amount of fuel for a given amount of air (a richer mixture) will deplete a greater part of the available oxygen. The O2 sensor in the exhaust pipe responds to this condition by raising the output voltage. Thus, the O2 sensor can help to maximize gas mileage and minimize the emission of pollutants. However, a typical O2 sensor has poor sensitivity in the range needed for acceleration, where the typical air/fuel ratio used in most cars is 12.5:1. Conventional O2 sensors are also sensitive to heat. Meaningful sensor output results only when exhaust temperatures are between approximately 360xc2x0 C. and approximately 900xc2x0 C.
The presence and concentration of gaseous oxide compounds have been measured using electrochemical sensing devices and methods which can generally be classified as either oxygen pumping sensors or potentiometric sensors. For example, U.S. Pat. No. 4,005,001 to Pebler, U.S. Pat. No. 4,770,760 to Noda et al., U.S. Pat. No. 4,927,517 to Mizutani et al., U.S. Pat. No. 4,950,380 to Kurosawa et al., U.S. Pat. No. 5,034,107 to Wang et al., and U.S. Pat. No. 5,034,112 to Murase et al and U.S. Pat. No. 5,217,588 to Wang disclose sensors for identifying presence and concentration of gaseous oxide compounds. Oxygen pumping sensors are amperometric sensors which xe2x80x9cpumpxe2x80x9d O2 through the cell at a rate proportional to electrical current induced in the pumping cell. However, most of the sensors referenced above are potentiometric sensors. Potentiometric sensors operate without xe2x80x9cpumpingxe2x80x9d and generate a voltage rather than an output current.
For example, Wang discloses a sensor formed from two electrochemical cells on a zirconia electrolyte. One cell senses only oxygen gas and the other cell senses all the gases which contain oxygen, including the oxygen gas. Both electrochemical cells are exposed to the same gas mixture, and the differences between the sensed signals is a measure of the concentration of NOx in the gas mixture.
Murase et al. discloses a sensor in which a catalyst for reducing NOx is placed on an electrolyte adjacent to a pumping cell. A current is induced in the pumping cell to control the oxygen concentration in the environment around the pumping cell. When the oxygen concentration is depleted to a predetermined level, the catalyst supposedly begins to deplete NOx, and the oxygen concentration of NOx is determined by measuring the current supplied to the pumping cell.
While pumping type sensors can be used to pump O2 from NO to form N2 and O2, they cannot generally be used to pump O2 from CO since C is not a gas and will deposit as a solid. Regarding potentiometric sensors such as the sensor disclosed by Wang, these sensors do not provide accurate measurement of CO or other oxide compounds in gas mixtures, because the electrodes used for the electrochemical cells are not sufficiently selective with respect to oxygen and oxide compounds, such as CO and NO. Moreover, if the gas mixture contains a relatively low oxide concentration compared with that of oxygen, an accurate determination of the oxide concentration is difficult. In exhaust gases or emissions produced by internal combustion engines or furnaces, the concentration of oxygen is typically much higher than the CO concentration. Thus, it is difficult to accurately measure the CO concentration in these gas mixtures using the typical pumping cell.
Another type of sensor described in U.S. Pat. No. 5,397,442 to Wachsman seeks to obviate this problem by providing a sensor including a chamber designed to receive a gas mixture in which two electrochemical cells are situated. Each cell is comprised of an electrode housed inside the chamber and an electrode outside the chamber, in which the internal and external electrodes are separated by an oxygen ion-conducting solid electrolyte. The first electrochemical cell is designed to consume oxygen by electrochemical reduction without appreciably consuming NOx, while the second electrochemical cell is relatively selective for the electrochemical reduction of NOx. A potential difference is applied across the first cell so that oxygen is removed from the chamber and then an electrical characteristic (voltage, current, power, etc.) of the second cell is measured that corresponds to the concentration of the oxide in the gas mixture. However, this system is somewhat complex and, because entry of gas into the chamber is diffusion limited, the response time of the sensor can be relatively slow.
A solid state electrochemical cell for measuring the concentration of a component of a gas mixture includes a first semiconductor electrode and a second semiconductor electrode, the electrodes comprising first and second semiconductor materials, respectively. The electrode materials are selected so as to undergo a change in resistivity upon contacting the component. A change in resistivity of the electrode materials results in a change in voltage across the electrochemical cell. An electrolyte is disposed in contact with the first and second semiconductor electrodes. The electrochemical cell can include a reference electrode in contact with the electrolyte.
At least one metal layer can be disposed on a portion of the semiconductor electrodes. The electrochemical cell can also include a detector for measuring an electrical characteristic generated by the electrochemical cell.
The semiconductor materials can include a metal oxide. The metal oxide is preferably SnO2, TiO2, TYPd5, MoO3, ZnMoO4 or WR3, where TYPd5 and WR3 are acronyms defined below. The acronym TYPd5 is used herein to represent a composite prepared by selecting TiO2 (titania), Y2O3 (yttria) and Pd in a weight ratio of approximately 85:10:5. Anatase titania is mixed with yttria and Pd metal powder in the composition described above. The powder is then applied onto the solid electrolyte in a slurry, and then sintered at approximately 650xc2x0 C. for 1 hr.
The acronym WR3 will be used herein to represent a composite which can be formed from the decomposition of Rh2WO6 at temperatures above approximately 1130xc2x0 C. into WO3 and metallic Rh. Oxygen is liberated in the decomposition reaction leaving a mixture of WO3 and 2Rh.
By selecting a first semiconductor material that exhibits a voltage response opposite in slope direction, the response being a function of detected gas concentration, to that of the second semiconductor material, the resulting voltage signal measured across the electrodes is substantially equal to the sum of the absolute values of individual voltage responses of the electrodes. The gas component measured can include CO.
The electrolyte is preferably an oxygen ion-conducting electrolyte. The oxygen ion-conducting electrolyte can be based on ZrO2, Bi2O3 or CeO2. Preferred oxygen ion-conducting electrolytes are electrolyte mixtures, the mixtures generally including a base material, such as ZrO2, Bi2O3 or CeO2 and one or more dopants, such as calcia (CaO) and yttria (Y2O3) which can function as stabilizers, or some other suitable oxygen ion-permeable material. For example, yttria stabilized zirconia (YSZ) electrolytes can be formed by mixing yttria and ZrO2. Electrolytes that conduct ionic species other than oxygen ions, e.g., halides, are well known in the art and also find utility in the invention for measuring halogen-containing gas species.
A solid state electrochemical cell for measuring the concentration of a component of a gas mixture includes a first semiconductor electrode and a second semiconductor electrode, the electrodes comprising first and second semiconductor materials, respectively, the materials selected so as to undergo a change in resistivity upon contacting the component. The first semiconductor material is selected to exhibit a voltage response opposite in slope direction, the response being a function of detected gas concentration, to that of the second semiconductor material, whereby a voltage signal measured across the electrodes is substantially equal to the sum of the absolute values of individual voltage responses of the electrodes. An electrolyte is disposed in contact with the first and second semiconductor electrodes.
A solid state electrochemical apparatus for measuring the concentration of at least two components of a gas mixture includes a plurality of electrochemical cells, the electrochemical cells each formed by two semiconductor electrodes. The semiconductor electrodes are formed from semiconductor materials, the materials selected so as to undergo a change in resistivity upon contacting at least one of the components in the gas mixture. An electrolyte is disposed in contact with the first and second semiconductor electrodes. At least one metal layer can be disposed on a portion of the semiconductor electrodes. The electrochemical apparatus can include a detector for measuring an electrical characteristic generated by the electrochemical cell.
At least one of the semiconductor materials can include a metal oxide, such as La2CuO4, SnO2, TiO2, TYPd5, MoO3, ZnMoO4 or WR3. At least one electrochemical cell can include a first electrode comprising a first semiconductor material having a voltage response opposite in slope direction, the response being a function of detected gas concentration, to that of the second semiconductor material, whereby a voltage signal measured across the first and second electrodes is substantially equal to the sum of the absolute values of individual voltage responses of the first and second semiconductor electrodes. The components measured can include CO and NO.
The electrolyte is preferably an oxygen ion-conducting electrolyte. The oxygen ion-conducting electrolyte can be based on ZrO2, Bi2O3 or CeO2. The electrochemical apparatus can include a reference electrode in contact with the electrolyte.
An electrochemical apparatus for measuring the concentration of a component of a gas mixture includes a plurality of electrochemical cells connected in series, the electrochemical cells each having a first electrode and a second electrode. At least one of the electrodes includes a material selected so as to undergo a change in resistivity upon contacting the component. An electrolyte is disposed in contact with the first and second electrodes. At least one of the electrodes in the plurality of electrochemical cells can include metal oxide semiconductor materials, such as La2CuO4, SnO2, TiO2, TYPd5, MoO3, ZnMoO4 or WR3. For CO detection, opposing electrodes in cells can both be metal oxide semiconductor materials, the metal oxide materials selected from SnO2, TiO2, TYPd5, MoO3, ZnMoO4 or WR3.
The electrochemical apparatus can include at least one metal layer disposed on a portion of the metal oxide-semiconductor materials. The electrochemical apparatus can include a detector for measuring an electrical characteristic generated by the electrochemical apparatus.
Preferably, cells having two semiconducting electrodes are formed from a first metal oxide semiconductor material which exhibits a voltage response being a function of detected gas concentration opposite in slope direction to the response of the second metal oxide semiconductor material, whereby a voltage signal measured across the electrodes is substantially equal to the sum of the absolute values of individual voltage responses of the electrodes. The measured component can include CO and NO.
The electrolyte is preferably an oxygen ion-conducting electrolyte, such as electrolytes based on ZrO2, Bi2O3 and CeO2. The electrochemical apparatus can include a reference electrode in contact with the electrolyte.
A solid state electrochemical cell for measuring the concentration of CO in a gas mixture includes a first semiconductor electrode, the first semiconductor electrode including a first semiconductor material selected so as to undergo a change in resistivity upon contacting CO. The first semiconductor material can be TiO2, TYPd5, MoO3, ZnMoO4 or WR3. A second electrode and an electrolyte is provided, the electrolyte in contact with the first and second electrodes.
The second electrode can preferably include a metal oxide semiconductor material such as SnO2, TiO2, TYPd5, MoO3, ZnMoO4 or WR3. At least one metal layer can be disposed on at least a potion of the semiconductor electrode materials. The electrochemical apparatus can include a detector for measuring an electrical characteristic generated by the electrochemical apparatus. A reference electrode can also be disposed in contact with the electrolyte.
A method for measuring the concentration of CO in a gas mixture includes the steps of exposing the gas mixture to a solid state electrochemical cell. The electrochemical cell is formed from (i) a semiconductor electrode, the semiconductor electrode comprising a semiconductor material, the semiconductor material selected so as to undergo a change in resistivity upon contacting CO, wherein the semiconductor material can include at least one selected from the group of materials consisting of TiO2, TYPd5, MoO3, ZnMoO4, WR3; (ii) a second electrode, and (iii) an electrolyte in contact with the first and second electrodes. An electrical signal generated by the electrochemical cell is measured to determine. the concentration of the component. The second electrode can also be formed from a semiconductor electrode.
When two semiconducting electrodes are provided, the first semiconductor material can preferably be selected to exhibit a voltage response opposite in slope direction, the response being a function of detected gas concentration, to that of the second semiconductor material, whereby a voltage signal measured across the electrodes is substantially equal to the sum of the absolute values of individual voltage responses of the electrodes. At least one of the semiconductor materials can include a metal oxide. The metal oxide can be SnO2, TiO2, TYPd5l MoO3, ZnMoO4 or WR3. The component measured can include CO.
A method for operating a combustion process, such as an engine, includes the steps of electrochemically determining the concentration of at least one exhaust pollutant emitted by the combustion process during operation, and adjusting combustion conditions based on concentrations of the exhaust pollutant determined in the determining step. The method can include the step providing an electrochemical cell, the electrochemical cell including (i) a first semiconductor electrode, (ii) a second semiconductor electrode, the electrodes comprising first and second semiconductor materials, respectively, the materials selected so as to undergo a change in resistivity upon contacting the pollutant, and (iii) an electrolyte in contact with the first and second semiconductor electrodes.
Rather than providing a single electrochemical cell, an electrochemical apparatus including a plurality of electrochemical cells can be provided. In one embodiment, the plurality of electrochemical cells can detect at least two of the exhaust pollutants, the plurality of electrochemical cells formed from first and second semiconductor electrodes, respectively, the electrode materials selected so as to undergo a change in resistivity upon contacting the pollutants. An electrolyte is provided in contact with the respective first and second semiconductor electrodes.
The method can include the step providing an electrochemical cell stack including a plurality of electrochemical cells connected in series, the electrochemical cells each including a first electrode and a second electrode. At least one of the electrodes is formed from a material selected so as to undergo a change in resistivity upon contacting the pollutant. An electrolyte is provided in contact with the first and second electrodes of respective electrochemical cells.
The method can include the step of providing an electrochemical cell, the electrochemical cell including (i) a semiconductor electrode, the semiconductor electrode comprising a semiconductor material selected so as to undergo a change in resistivity upon contacting CO, wherein the semiconductor material is TiO2, TYPd5, MoO3, ZnMoO4 or WR3 and (ii) a second electrode. An (iii) electrolyte is provided in contact with the semiconductor electrode and the second electrode. The second electrode can be a semiconducting electrode.
Internal combustion engines, such as those included in motor vehicles, can utilize the invention. For example, an internal combustion engine can include an at least one cylinder, the cylinder for combusting a fuel mixture therein, the engine emitting a gas mixture comprising a plurality of pollutants. An electrochemical emission sensor is disposed to receive the emitted gas mixture and for determining the concentration of at least one of the plurality of pollutants. A feedback and control system is provided for receiving pollutant gas concentration data from the emission sensor and for directing adjustment of engine combustion conditions. The emission sensor can include an electrochemical cell, the electrochemical cell formed from a first semiconductor electrode and a second semiconductor electrode, the electrodes including first and second semiconductor materials, respectively. The electrode materials are selected so as to undergo a change in resistivity upon contacting the pollutants. An electrolyte is disposed in contact with the first and second semiconductor electrodes. Alternatively, the emission sensor can include an electrochemical apparatus, the electrochemical apparatus including a plurality of electrochemical cells, the electrochemical cells formed from (i) a first semiconductor electrode, (ii) a second semiconductor electrode, and an electrolyte in contact with the first and second semiconductor electrodes of respective electrochemical cells.
An electrochemical apparatus can include a plurality of electrochemical cells connected in series, the electrochemical cells including a first electrode and a second electrode, at least one of the electrodes comprising a material selected so as to undergo a change in resistivity upon contacting the pollutants, and an electrolyte in contact with the first and second electrodes of respective electrochemical cells. The emission sensor can include an electrochemical cell, the electrochemical cell including (i) a first semiconductor electrode, the semiconductor electrode comprising a first semiconductor material selected so as to undergo a change in resistivity upon contacting CO, wherein the first semiconductor material is TiO2, TYPd5, MoO3, ZnMoO4 or WR3, (ii) a second electrode, and (iii) an electrolyte in contact with the first and second electrodes.
A method of forming a solid state electrochemical cell for measuring the concentration of a component of a gas mixture includes the steps of forming a first semiconductor electrode and a second semiconductor electrode, the electrodes comprising first and second semiconductor materials, the materials selected so as to undergo a change in resistivity upon contacting the component. An electrolyte is formed, the electrolyte being in contact with the first and second semiconductor electrodes.
A method for controlling a chemical process can include the steps of providing an electrochemical cell including (i) a first semiconductor electrode and a (ii) a second semiconductor electrode, the electrodes comprising first and second semiconductor materials, respectively. The materials are selected so as to undergo a change in resistivity upon contacting gas emitted by the chemical process. An (iii) electrolyte is provided in contact with the first and second semiconductor electrodes. The concentration of at least one gas emitted during operation of the chemical process is electrochemically determined. Chemical process conditions are adjusted based on concentrations of the gas determined in the determining step. The chemical process can be a combustion process. Preferably, the first semiconductor material exhibits a voltage response opposite in slope direction, the response being a function of detected gas concentration, to that of the second semiconductor material, whereby a voltage signal measured across the electrodes is substantially equal to the sum of the absolute values of individual voltage responses of the electrodes.
A solid state electrochemical cell for measuring the concentration of NO in a gas mixture includes a first semiconductor electrode comprising La2CuO4, a second electrode and an electrolyte in contact with the first and second electrodes. The second electrode can include Pt, while the electrolyte can comprise ZrO2, Bi2O3 or CeO2. A method for measuring the concentration of NO in a gas mixture includes the steps of exposing the gas mixture to a solid state electrochemical cell, the electrochemical cell formed from (i) a semiconductor electrode comprising La2CuO4 which undergoes a change in resistivity upon contacting NO, (ii) a second electrode, and (iii) an electrolyte in contact with the first and second electrodes. An electrical signal generated by the electrochemical cell is measured and used to determine the concentration of NO in the gas mixture.